1. Field of the Invention
The present invention relates to a prosthetic cardiac valve intended to be implanted in a human being for replacing a natural cardiac value which has been damaged or functions badly.
2. Description of the Prior Art
Several cardiac valve prostheses have been proposed in the Prior Art. In the work by E.A. LEFRAK and A. STARR, edited in 1979 by Appleton-Century-Crofts, New York, and entitled: "Cardiac valve prostheses", there are presented the historical, hemodynamic and clinical aspects of different mechanical and biological prosthetic cardiac valves. Among the mechanical cardiac values (to which our analysis of the Prior Art refers, because biological prothetic valves dapart from the scope of the invention), there will only be recalled here some solutions comprising at least two flaps letting the blood flow in one direction and preventing it from flowing in the opposite direction.
The American patent S. MILO No. 3 938 197, describes an artificial cardiac value with central flow, namely having the central zone completely cleared in the open position. This value comprises a plurality of flaps, more particularly five, which have a substantially triangular shape and whose lower face is concave over about the lower 4/5 of its height, whereas it is convex over about the upper 1/5 of said height, which flaps are each hinged about a peripheral segment formed by the male/female assembly of two elements, which assembly is stabilized by means of a screw which is fitted into a peripheral hole formed in an element parallel to the pivoting axis of the corresponding flap and which is screwed into the other element.
The American patent Z. C. POSSIS No. 4 078 268 describes a prosthetic cardiac valve with, in a first embodiment, two flaps each pivoting about a shaft integral with the corresponding flap and formed by two shaft projections disposed on the lateral sides of the valve at a distance from its leading edge equal to 1/3 of the height of the value, the rotational movement of each flap being limited by appropriate projections projecting from the supporting rim (or base or body) of the flaps, towards the central passage of the valve.
In a second embodiment, the above POSSIS patent describes valves whose rotational movement between the closed and open positions and vice-versa is provided, under the action of the cardiac pulsation, by pivoting and stop means each having two pairs of projections which project slightly from the annular base or housing of the valve towards the central passage and which define together a virtual axis of rotation. Each pair of projections is formed by an upper projection having a pivot portion and a substantially vertical stop face and by a lower projection slightly offset with respect to the upper portion and it is also provided with a pivot portion and a stop face inclined inwardly and upwardly. Each valve is included between two pairs of projections, the upper projection of which cooperates with a lateral lug projecting from the upper face of the corresponding valve and disposed below with respect to said upper projection.
Insofar as the support base for the flaps described in the POSSIS patent is concerned, it is formed by screwing or fitting/clipping an internal portion in an external portion, the internal portion being possibly formed by two semi-circular segments in the case where the assembly is provided by fitting/clipping together.
In addition, the distance of each of the two real or virtual axes of rotation, with respect to the axis of the valve, is between about 75% and 65% of the radius of the central passage defined by the support base, so that this latter is amply freed without however being optimally freed.
The French patent CARPENTIER No. 2 331 997, corresponding to the American patent No. 4 159 543, describes a prosthetic cardiac valve of the type including a substantially circular support base having at least one seat and two substantially semi-circular movable flaps adapted to rest on the seat.
Each flap of the CARPENTIER valve cooperates with means allowing the translational and rotational movements of the two flaps, and particularly the rotation through an angle less than 90.degree., and translation parallel to the axis of the support base. The roto-translational means are formed, for each flap, by a transverse shaft disposed in the vicinity of a diameter of the base and by pegs, particularly in the form of shaped ridges, disposed alternately on each side with respect to the corresponding shaft and projecting from the external face of the valve. The translational movement is provided by the fact that, when the valve is considered in the closed position and therefore with each flap resting of the corresponding seat, between the shaft of rotation and the external face of the flap, there exists a certain distance, which prevents the permanent contact between each flap and the corresponding shaft.
There are also known from the French patent application No. 2 543 429, in the name of UNIVERSITE PIERRE ET MARIE CURIE and having again as inventor M. CARPENTIER, some improvements which are brought about the very prosthetic cardiac valve described in the above CARPENTIER French patent No. 2 331 997: it is a question of providing each flap, and/or the corresponding seat of the valve, of a device known per se, capable of ensuring controlled opening of the valve when the blood pressure is balanced on both the internal and external faces of the mobile flap; that means that, under these conditions of pressure balance, the valve is slightly open. The device, known per se, proposed by CARPENTIER for controlling the opening of the valve is formed, for each flap, by two magnets or electro-magnets one of which is disposed on the flap whereas the other is disposed on the corresponding seat. Alternately, this device may be formed by a spring fixed to the seat.
Furthermore, the American patent No. 4,276,658 D. W. HANSON et al, corresponding to French patent No. 2 407 709, describes a cardiac valve prosthesis of the type having a body with a central passage for blood flow, means for adjusting this flow, which pivot under the action of the blood flow between a closed position and an open position, and means for hinging these adjustment means. The HANSON et al valve includes hinging means having cavities formed in said body and with a bearing surface in the form of a surface of revolution and adjustment means provided with projections for penetrating into the cavities and sweeping the (abutment) surface with the end which is in contact with this surface.
The American patent L. C. MEYER, No. 3 589 392, is also known which describes an artificial cardiac valve whose flaps have a concave internal face (substantially in the form of a spoon) and are hinged (pivoted) about a flexible arcuate portion which is supported by the base of the valve; however, the MEYER valve has a dimension in height which is exaggerately large, which is due particularly to the design of the pivot means used for the flaps.
Apart from the MEYER valve for the above reasons, and in particular because it is not a valve of the so-called "low profile" type, it is clear that each of the prosthetic cardiac valves of the Prior Art have advantages and disadvantages which are proper thereto, particularly:
insofar as the MILO valve is concerned, though on the one hand it provides a central flow for the blood, on the other hand it has zones of stagnation of the blood under said peripheral hinging segments of the flaps, namely that the cellular elements of the blood tend to remain confined under these segments, which produces a beginning of coagulation and therefore, in the long run, thrombosis effects;
in so far as the POSSIS valve is concerned, although it eliminates the problems of stagnation of the blood occurring in the MILO valve, however the central blood flow passage is not yet freed to the maximum of the possibilities and the flaps are slanted towards the valve axis, in the open position, so that the problems of turbulence--even though they are reduced--are still considerable; furthermore, the constructive solution of the support base for the flaps formed of several elements presents the potential risk of separation of these elements and of the formation of blood stagnation zones;
insofar as the CARPENTIER valve is concerned, described in the French patent No. 2 331 997, although it avoids the formation of permanent contacts between the fixed parts and the movable parts of the prosthesis and so reduces the formation of clots, on the other hand the substantially central position of the flaps and their relatively considerable slant with respect to the vertical are sources of turbulences in the central part of the flow, which turbulences are all the greater and more troublesome since the flow is faster at this position, so that the preceding advantage is practically cancelled out and the risks of coagulation exist also in this case;
insofar as the CARPENTIER valve is concerned described in the French patent No. 2 543 429, it should be emphasized that it privileges the performances at opening time, whereas, in practice, it is rather the behavior of the valve at closing time which raises problems, particularly due to the blood regurgitation during the return of the blood flow in the reverse direction;
insofar as the HANSON et al valve is concerned, the harmful effects (crushing and so destruction) on the red corpuscles of the blood still remain considerable because of the friction action between said projections of the flaps and said cavities housing these projections.
Furthermore, in a general way, the design of all the prosthetic valves of the Prior Art is such that the hinging means are subjected to very high forces which are responsible for the premature wear of the existing valves. An object of the present invention is therefore to provide a prosthetic valve whose hinge means are subjected only to low forces, thus taking care of a vital aspect in the design of cardiac valve protheses, as is rightly emphasized by the surgeon D. LIOTTA of the HOSPITAL ITALIANO of Buenos Aires, in Argentina, on page 49 of the TEXAS HEART INSTITUTE JOURNAL, vol 12 No. 1 of March, 1985.